REGULATION OF MURINE ACID-SECRETION BY CO2

To determine whether endogenous metabolic sources alone provide suffic ient CO2 for acid secretion in mammals, basal and stimulated acid secr etion and metabolic CO2 production were measured concurrently in mouse stomachs, in vitro, without exogenous CO2, and after addition of 5% C O2 serosally. Basal acid secretion was varied by changing luminal pH f rom 3.2 to 4.0. In the absence of an exogenous supply of CO2 acid secr etion was stable under basal conditions and increased during cholinerg ic stimulation with carbachol. Serosal CO2 supply increased basal and stimulated acid secretion. The increase in basal acid secretion depend ed on the initial level of acid secretion. At pH 4.0, exogenous CO2 in creased acid output (mean +/- SD) by 13% from 112 +/- 11 nmol/min to 1 26 +/- 8 nmol/min (P < 0.03), whereas at pH 3.6 the increase was 40% ( 63 +/- 14 to 88 +/- 20 nmol/min, P < 0.04) and 157% at pH 3.2 (21 +/- 13 to 54 +/- 14 nmol/min, P < 0.002). Following cholinergic stimulatio n a maximal acid output of 321 +/- 38 nmol/min was attained without se rosal CO2 whilst addition of 5% CO2 to the serosal solution increased maximal acid secretion by 49% to 479 +/- 96 nmol/min (P < 0.005). Meta bolic activity, measured as total gastric CO2 production, was greater as acid secretion rates increased [239 +/- 20 nmol/min at 21 +/- 13 nm ol/min (luminal pH 3.2) versus 406 +/- 28 nmol/min at 321 +/- 17 nmol/ min (after cholinergic stimulation)]. The data support the concept tha t basal and submaximal acid secretion can be maintained by CO2 availab le from metabolic sources, but full expression of the acid secretory a pparatus requires exogenous CO2.